The present invention relates to a liquid crystal display device utilizing a polarized light, and in particular, to a liquid crystal display device having high brightness and high contrast.
The liquid crystal element of the liquid crystal display device has a low consumption power because it is a non-luminous element for modulating external light. Furthermore, it can be applied to a flat display because of its thin thickness and light weight. Accordingly, taking advantage of the aforementioned merits, the liquid crystal element is used in a watch, a desk-top calculator, a computer terminal, a word processor, a television receiver or the like so as to be utilized as an information display device in a wide variety of fields.
The modern society is called the advanced information society, where an increasing amount of information is communicated and individuals have growing demands for collecting and selecting information. On such a social background, there have been an extensive recognition of a necessity for a personal use portable information terminal as well as an expectation for the implementation of it, along which the development of such a terminal has been positively pursued.
Lately, there is a demand for, in particular, a reflection type liquid crystal display device which has high brightness and high contrast and of which the background and the color of tone at the time of an off-state voltage are white like a paper and the black level at the time of an on-state voltage looks like a trace of a pencil tip on a quality paper.
However, according to the conventional transmission type, semi-transmission type and reflection type liquid crystal display devices, the incident light is made to outgo after passing through a polarizing plate at least two times as follows.
That is, the transmission type liquid crystal display device is generally constructed by arranging a liquid crystal element between a pair of polarizing plates. Then, incident light entering from one polarizing plate side is made to outgo through the other polarizing plate. The reflection type liquid crystal display device is constructed by laminating a polarizing plate, a liquid crystal element, a polarizing plate and a reflecting plate in the above-mentioned order. Then, incident light from the outermost polarizing plate is reflected on the reflecting plate and made to outgo through the outermost polarizing plate.
As both the polarizing plates that interpose the liquid crystal element between them, a macromolecular dichroic polarizing plate has been widely put into practical use. This polarizing plate transmits the incident light only in a specified direction of the polarizing plane and almost absorbs the remaining light. In this case, theoretically 50% of the incident light is lost, and this fact becomes the biggest problem in increasing the brightness of the LCD (Liquid Crystal Display) panel. Particularly, in the case of the reflection type liquid crystal display device, the incident light is made to outgo after passing through the polarizing plates four times, and therefore, the aforementioned problem becomes more serious. Furthermore, a shade is generated by a parallax due to a distance from the liquid crystal layer to the reflecting plate, consequently causing a significant reduction in visibility.
Accordingly, a variety of proposals for increasing the brightness by resolving the reduction in visibility due to the parallax of the reflection type liquid crystal display device have been made.
As a reflection type liquid crystal display device as described above, there is a one disclosed in, for example, the document of Japanese Patent Laid-Open Publication No. HEI 6-230362. This liquid crystal display device is a reflection type TN (Twisted Nematic) liquid crystal display device.
On one side of the liquid crystal element is arranged a polarizing plate. On the other side of the liquid crystal element are provided a quarter-wave plate, a cholesteric liquid crystal polymer film and an optical absorption plate in order from the liquid crystal element side while being arranged tightly close to one another. Then, an unnecessary light entering from the polarizing plate side is absorbed by the optical absorption plate, thereby making the light in a specified range of wavelength pass again and outgo through the polarizing plate. That is, by using the aforementioned quarter-wave plate and the cholesteric liquid crystal polymer film as the polarizing plate on the other side and using the optical absorption plate in place of the reflecting plate for the absorption of the unnecessary light, only the reflected light from the liquid crystal element becomes the outgoing light. Therefore, by virtue of the removal of the shade, the reduction in visibility is eliminated, and the brightness is increased by reducing the number of polarizing plates to one.
The document of Japanese Patent Laid-Open Publication No. HEI 1-133003 discloses a high-performance polarizing plate which prevents the absorption of light. This high-performance polarizing plate is formed by laminating the quarter-wave plate with a macromolecular liquid crystal layer comprised of a cholesteric phase.
However, the aforementioned prior art liquid crystal display device utilizing the cholesteric liquid crystal polymer film (Japanese Patent Laid-Open Publication No. HEI 6-230362), the cholesteric liquid crystal polymer film reflects only the light in the specified range of wavelength. Although the quarter-wave plate has an ability of converting light of a specified wavelength into a linearly polarized light, it is impossible to convert light in the whole visible wavelength region into the linearly polarized light. The above results in the problem that the outgoing light is disadvantageously colored, hindering the achievement of the paper-white appearance.
Also, in the case of the liquid crystal display device (Japanese Patent Laid-Open Publication No. HEI 1-133003) which uses the high-performance polarizing plate formed by laminating the quarter-wave plate with the macromolecular liquid crystal layer comprised of the cholesteric phase, the outgoing light is transmitted through the cholesteric liquid crystal and the quarter-wave plate similar to the aforementioned case, hindering the achievement of the paper-white appearance.
Therefore, in the liquid crystal display device disclosed in the document of Japanese Patent Laid-Open Publication No. HEI 8-271731, a multi-phase cholesteric film is laminated with a plurality of quarter-wave plates having different wavelengths of light to be converted into a linearly polarized light, thereby allowing the light in the whole visible wavelength region to be converted into the linearly polarized light. However, this liquid crystal display device has a problem that the transmittance is reduced due to the multiplicity of the layers to be laminated.